Basic questions to consider in deployment sizing:
Total Number of Users?
Number of Active Users? (checking email, calendar, contacts, etc. at least once a day)
Number of Concurrent Users? (logging into the UI, checking email, calendar, contacts during Message Delivery Peak Period (see question below)
What percentage of total users will be using Webmail vs POP vs IMAP vs Mobile vs Outlook?
For example 60% Webmail, 50% IMAP, 10% POP (I don't care if this adds up to 100% of the 10k user count, ie we can consider the fact that users may be accessing ZCS via multiple different clients)
GAL lookups being done from those IMAP clients?
Message Delivery Peak Period in minutes?
Period of time in minutes where message delivery (inbound + outbound) is the highest (i.e. from 10am to 12pm then 120 minutes)
Percentage Message Delivery Peak Workload?
What percentage of workload is occurring during the message delivery peak period(s) (i.e. 80% of workload occurs from 10am to 12pm)
Mailbox Quota per user (MB)?
Average Mailbox Utilization Percentage % of Quota? (how much of that quota will they generally use) or Average Mailbox Size (MB)?
Will you be using the Zimbra AS/AV on Zimbra MTAs?
If no to above, would you have your third party Anti-Spam appliances querying Zimbra LDAP server to check on unknown recipients?
Total Incoming messages per day?
Total Prespam Incoming Messages per day? (Prespam = SPAM + LEGIT)
Total Postspam Incoming Messages per day? (Postspam = LEGIT)
Number of Postspam Incoming Messages per day? (Postspam = LEGIT)
Total Outgoing messages per day?
Number of Outging messages per user per day?
Average Message Size (KB)?
Percentage % of Messages with attachments?
Average Attachment Size (KB)?
Disks? On-box storage (SCSI vs SATA) vs NAS vs SAN (& Fiber vs iSCSI etc etc) Use raid RAID10 not RAID5 for faster storage. Writes are slower in RAID5
& you are talking about lots of tiny files. Lots of threads on disks if you search. (The NE version has Hierarchical Storage Management which offloads mail after x days to your cheaper/older disks & RAID5 is usually fine for that.)